Traditional Telephony

Voice Systems Rely on Public Switched Telephone Networks

In a typical voice/analog telephone network, users make an outside phone call from the phone on their desk. The call then connects to the company’s internal phone system or directly to the Public Switched Telephone Network (PSTN) over a basic telephone service analog trunk or a T1/E1 digital trunk. From the PSTN, the call is routed to the recipient, such as an individual at home.

If a call connects to a company’s internal phone system, the call may be routed internally to another phone on the corporate voice network without ever going through a PSTN. The PSTN may contain a variety of transmission media, including copper cable, fiber-optic cable, microwave communications, and satellite communications.

Traditional Telephony Equipment

A telephone set is simply a telephone.

KTS: Key telephone systems, found commonly in small business environments, enhance the functionality of telephone sets. The telephones have multiple buttons and require the user to select central-office phone and intercom lines. EKTS: Electronic key telephone systems improve upon KTS systems. EKTSs often provide switching capabilities and impressive functionality, crossing into the PBX world.

PBX: A private branch exchange system allows the sharing of pooled trunks (outside lines) to which the user typically gains access by dialing an access digit such as “9.” Software in the PBX manages contention for pooled lines. The PBX system has many features, including simultaneous voice call and data screen, automated dial-outs from computer databases, and transfers to experts based on responses to questions rather than phone numbers.

The historical differences between a PBX and a key system have blurred, and both product lines offer comparable feature sets for station-to-station calling, voice mail, and so on. Either the customer owns the PBX or it can be owned and operated by a third party as a service to the end customer. To blur things further, key systems are beginning to offer selected trunk interfaces.

The major differences between a PBX and a key system are the following:

– A PBX looks to the network like another switch—it connects via trunk (PBX-to-PBX) interfaces to the network.

– A key system looks like a phone set (station) and connects via lines (station to PBX).

– PBXs serve the high end of the market.

– Key systems serve the low end of the market.

CO: The central office is the phone company facility that houses the switches.

Switch: An electromechanical device, a switch performs the central switching function of a traditional telephony network. Today, it can include both analog and digital hardware and software.

Toll switch: This switch is used to handle long-distance traffic.

Traditional TelephonySignaling, Addressing, and Routing

We will now consider how phone calls are created and sent through the traditional telephone network Signaling

– Off-hook signaling – how a phone call gets started

– Signaling paths

– Signaling types

Addressing

– Very different from data network schemes

– These differences must be resolved in order to implement integrated data/voice/video (DVV)

Routing

– Dependent on the resolution of the addressing issue

Signaling in a Voice System Sets Up and Tears Down Calls

In any telephone system, some form of signaling mechanism is required to set up and tear down calls. When a caller from an office desk calls someone across the country at another office desk, many forms of signaling are used, including the following:

– Between the telephone and PBX

– Between the PBX and CO

– Between two COs

All of these signaling forms may be different. Simple examples of signaling include ringing of a telephone, dial tone, ringing, and so on.

There are five basic categories of signals commonly used in a telecommunications network:

Supervisory—Used to indicate the various operating states of circuit combinations. Also used to initiate and terminate charging on a call.

Information—Inform the customer or operator about the progress of a call. These are generally in the form of universally understood audible tones (for example, dial tone, busy, ringing) or recorded announcement (for example, intercept, all circuits busy).

Address—Provides information about the desired destination of the call. This is usually the dialed digits of the called telephone number or access codes. Typical types of address signals are Dial Pulse (DP), DTMF, and MF.

Control—Interface signals that are used to announce, start, stop, or modify a call. Controls signals are used in interoffice trunk signaling.

Alert—Ringing signal put on subscriber access lines to indicate an incoming call. Signals such as ringing and receiver off-hook are transmitted over the loop to notify the customer of some activity on the line.